The invention relates to an optical body having modifiable light guiding properties, in particular an optical fiber, the light guiding properties of which can locally be changed by a suitable treatment, and to a method of producing an optical waveguiding body.
Several different methods of locally changing the light guiding properties such as the refractive index in an otherwise finished optical waveguide are previously known. Here can be mentioned:
1. Permanent increases of the refractive index in SiO2-based optical fibers doped with germanium oxide can be obtained by subjecting such fibers to ultraviolet radiation. This radiation should have wavelengths corresponding to the wavelengths of some absorption interval or intervals of germanium oxide(s). An example of the use of such changes of refractive index induced by ultraviolet light are fiber gratings, which are described e.g. in the document by Phillip St. Russel et al., xe2x80x9cFibre gratingsxe2x80x9d, Physics World, October 1993, pp. 41-46, and the published International patent application WO 94/00784. The produced gratings are for example utilized as filters. The grating structure of such gratings disappears, when they are exposed to temperatures higher tan 500-900xc2x0 C.
2. The increase of the refractive index of glass material doped with germanium oxide can be reinforced, by making, before the treatment with ultraviolet light, hydrogen diffuse into the material, so called hydrogen sensitization. The refractive index of glass material doped with germanium oxides, phosphorous or phosphorous-aluminium can also be increased by making first hydrogen diffuse into the material in a treatment in a hydrogen gas atmosphere at a high pressure and by thereupon heating the material at not too high temperatures, such as in an interval above 500xc2x0 C., see U.S. Pat. No. 5,500,031 for Atkins et al. corresponding to the published European patent application 0622343.
3. In the the Swedish patent application 9603406-1, filed Sep. 17, 1996, instead a chemical reaction induced by light is used to achieve such changes Then fluorine atoms are supposed to already initially exist in the core of the fiber, which atoms can be assumed to be bonded in the glass structure. To the fiber hydrogen atoms are added by making them diffuse into the fiber from a surrounding hydrogen gas atmosphere having a high pressure. The fiber is irradiated at selected areas with ultraviolet light in order to make the germanium atoms in the core together with hydrogen, which has diffused into the material, and the quarts material of the fiber form hydroxyl groups. The hydroxyl groups formed react with fluorine atoms to form hydrogen fluoride. Hydrogen fluoride is not bonded too strongly to the material but can by means of a suitable heating operation be made to diffuse from the core into the surrounding material. Thereby the concentration of fluorine in the core within the irradiated region is reduced, what increases the refractive index of the core within these regions. This method can be summarized by: Glass doped with fluorine is used in a fiber, which is first subjected to a hydrogen sensitization, thereupon to a UV-exposure and finally to a heating operation.
4. Dopants, for example germanium and fluorine atoms, which are initially arranged in a glass material, can diffuse in a heating operation, what can change the refractive index within portions exposed to heat. Germanium increases the refractive index of glass materials, whereas fluorine reduces the refractive index. Then, if germanium atoms for example exist within a limited region of an optical fiber, in which thus the refractive index is higher than in the surrounding material, the refractive index can be reduced in this limited region by a heating operation. Inversely the refractive index can by a heating operation be increased within a region having only fluorine atoms.
The methods according to 1. and 3. above are primarily used to produce grating structures, i.e. periodic changes of the refractive index in space, such as changes in the waveguiding core in an optical waveguide, for example changes of the refractive index of is the core in an optical fiber, which are periodic along the longitudinal axis of the fiber.
The method according to 2. above can be used to increase the refractive index of regions doped with germanium oxides, e.g. to produce waveguides in planar substrates and thus to allow the manufacture of integrated optical components.
The methods according to 1., 2. and 3. change the refractive index only within the regions, in which there initially is some concentration of germanium and/or of fluorine atoms. Surrounding, substantially undoped regions, such as for example the cladding of an optical fiber having a doped core region, are not generally noticeably influenced. The core in an optical waveguide maintains in these cases, during the processing, its extension, in particular its radius when the optical body considered is an optical fiber.
In the method according to 4. above very high temperatures must be used to produce a noticeable diffusion. In many typical waveguides, such as in conventional optical fibers intended for telecommunication, the increase of the refractive index in the portion, which is to form the very waveguide or the core of the waveguide, is produced by doping the glass material with germanium oxide GeO2 when producing the waveguides. When thus a conventional optical fiber is exposed to a high temperature of the magnitude of order of 1600xc2x0 C. during a not too short period of time, germanium atoms diffuse away out of the core region into the surrounding material, i.e. into the cladding. The difference between the refractive indices of a material doped with germanium and of a substantially undoped material, such as in the cladding, is proportional to the concentration of germanium atoms, what implies, that the diffusion gives a xe2x80x9csmearingxe2x80x9d of the refractive index, i.e. the core region is expanded and the refractive index thereof is reduced. The refractive index of a typical optical fiber before and after such a high temperature treatment, which also can be termed a core diffusion, is shown in the diagram of FIG. 7.
For an optical fiber the numerical aperture thereof is given by       NA    =                            n          1          2                -                  n          2          2                      ,
where n1 is the refractive index of the core of the optical fiber and n2 is the refractive index of the cladding, surrounding the core of the fiber. By a high temperature heating operation thus the numerical aperture of the fiber can be reduced and in particular such a heating operation can be used in the end region of the fiber when connecting it to other fibers or components.
In the articles by H. Y. Tam, xe2x80x9cSimple Fusion Splicing Technique for Reducing Splicing Loss Between Standard Singlemode Fibres and Erbium-doped Fibrexe2x80x9d, Electronics Letters, Aug. 15, 1991, Vol. 27, No. 17, and J. S. Harper et al., xe2x80x9cTapers in Single-mode Optical Fibre by Controlled Core Diffusionxe2x80x9d, Electronics Letters, Feb. 18, 1988, Vol. 24, No. 4 core diffusion is used to adapt mode fields when connecting optical fibers having different numerical apertures to each other or for adaption when connecting optical fibres having different diameters to each other. Core diffusion can also be used to adapt the mode fields in different xe2x80x9cfiber-to-fiber-componentsxe2x80x9d, see the articles by Kazuo Shiraishi et al., xe2x80x9cBeam Expanding Fiber Using Thermal Diffusion of the Dopantxe2x80x9d, J. Lightw. Teckin., Vol. 8, No. Aug. 8, 1991, and Kazuo Shiraishi et al., xe2x80x9cLight-Propagation Characteristics in Thermally Diffused Expanded Core Fibersxe2x80x9d, J. Lightw. Techn., Vol. 11, No. Oct. 10, 1993. Mode field adaption is also used in the methods described in U.S. Pat. Nos. 5,301,252, 5,142,603 and 5,381,503.
In the article by J. Kirchhof et al., xe2x80x9cDiffusion behaviour of fluorine in silica glassxe2x80x9d, J. of Non-Cryst. Solids 181, 1995, pp. 266-273, is among other things disclosed, that phosphorous atoms in a SiO2-material strongly favour the diffusion of fluorine atoms, i.e. that fluorine atoms in such a material obtain an increased mobility and diffuse much more easily, when also phosphorous atoms exist in the material. In contrast the existence of germanium atoms does not significantly influence the diffusion characteristics of fluorine. In the cited U.S. Pat. No. 5,500,031 for Atkins et al. is mentioned that the refractive index of a P-F doped ordinary cladding material increased significantly during a heating. The cladding material also contained hydrogen.
It is an object of the invention to provide optical structures, in particulair optical fibers, which can selectively be given modified waveguiding properties and which selectively maintain some of the modified properties even when they are subjected to very high temperatures.
It is another object of the invention to provide a method of producing an optical waveguiding structure, in particular an optical fiber, which by a local process is given modified waveguiding properties and which can maintain some of the modified properties even when it is subjected to very high temperatures.
Using suitably selected dopants and suitably selected concentrations thereof in an optically transparent ground mass such as in the cladding in an optical fiber and by a local heating operation the refractive index structure can be changed, whereby waveguides or waveguide cores can be xe2x80x9ccreatedxe2x80x9d or xe2x80x9cerasedxe2x80x9d and at the same time other existing structures comprising waveguide cores, if desired, can remain substantially unchanged. This can be obtained by using in for example an optical fiber of in principle a standard type for telecommunication that the diffusion properties of fluorine, which reduces the refractive index, are strongly changed, if also a doping with phosphorous is made, which increases the refractive index. Fluorine combined with phosphorous thus diffuses much more easily than only fluorine, when no phosphorous atoms exist, and much more easily than germanium and phosphorous. Hydrogen atoms are not required.
In specially designed optical fibers of type single mode fibers thus locally additional waveguide cores can be created parallel to the already existing waveguide core. In this way to some extent xe2x80x9cintegrated optical componentsxe2x80x9d can be locally created inside an optical fiber. There is also a possibility of erasing a waveguide core by a suitable initial choice of the dopings and doping levels.
The methods according to 1. and 3. and the method according to the first part of 2. above could be used to create a waveguide core, by exposing a region, which is so doped with germanium oxides and with materials reducing the refractive index such as boron oxides or fluorine, to ultraviolet light that before the treatment with ultraviolet light there is no refractive index difference in the glass material, i.e. no waveguide core exists. A disadvantage of these methods is that it is difficult and time consuming to obtain sufficient increases of the refractive index to produce a normal waveguide core. Further, when using these methods an existing waveguide core cannot be erased.
The method described under 4. can be used only to change a mode field of a waveguide, i.e. the very light distribution in the waveguide, and/or the diameter of the waveguides core. It can therefore hardly be used to create a new waveguide core and is too time consuming to erase an existing waveguide core.
By instead changing the temperature dependent diffusion properties of an atom or ion kind such as fluorine, which constitutes a doping in a glass material, by a sufficient addition of another atom or ion kind such as phosphorous, waveguide cores can be created or erased by a suitable heat treatment, whereas other waveguide cores, which do not contain any of these ion or atom kinds are not noticeably influenced. For the doping system fluorine-phosphorous a temperature in the heat treatment of about 1300xc2x0 C. is used and thus in this system and at his temperature fluorine, which exists in regions, which simultaneously have a suitable concentration of phosphorous, will diffuse out of these regions.
Thus generally, an optical waveguiding body is considered hereinafter, which contains substantially silicon dioxide and has waveguiding properties for guiding light of some wavelength or wavelength interval or band. Then the body can generally have at least one waveguide core, along which light of the wavelength can propagate. In the body a region is provided which is doped with at least fluorine and phosphorous to carefully determined concentrations. The region does in the preferred case not contain any hydrogen atoms which are free to move when heating the body and which thus are not strongly chemically bonded to the material of the body. The region can be a waveguide core which e.g. works as a standard core for letting light propagate therealong. The region can then contain an additional kind or additional kinds of atoms or ions, e.g. germanium.
The doping concentrations or doping levels of fluorine, phosphorus and possible other ions or atoms are so selected, that when heating a part of the region the refractive index of the heated part of the region for light of the considered wavelength is changed and in some cases even strongly changed. Then the light guiding properties of the heated part of the region for light of the considered wavelength will also be changed or strongly changed respectively. Differently selected concentrations and locations of the region, in particular of the heated part of the region will then produce a new waveguide core, erase an existing waveguide core, or change the waveguiding properties of the part which is heated so that light after the heating can or cannot be coupled thereto or therefrom. Typical cases comprise:
The region has before heating no waveguiding properties for light of the considered wavelength but the part of the region forms when heated or after being subjected to a heating process a new waveguide core capable of guiding therealong light of the considered wavelength.
The region has before heating some waveguiding properties for light of the considered wavelength but does not allow coupling of light between the region and another waveguide core in the body. For example the region can have the general shape of a waveguide core but have a refractive index different from that of the other waveguide core resulting in different propagation velocities of light which do not allow coupling of light therebetween. The refractive index of the region can e.g. correspond to a difference between this refractive index and the refractive index of the bulk material of the body, i.e. of the cladding in the case where the body is an optical fiber, the difference being about half the difference between the refractive index of the other waveguide guide and that of the bulk material. The part of the region obtains after heating changed waveguiding properties allowing a coupling of light between the other waveguide core and the heated part of the region.
The region has before heating waveguiding properties for light of the considered wavelength and works substantially as a normal waveguide core of the body. The part of the region looses, after heating, its waveguiding properties, i.e. a waveguide core of the body is made to disappear.
The region has before heating waveguiding properties for light of the considered wavelength and works substantially as a normal waveguide core of the body and allows a coupling of light to and from another waveguide core. The part of the region changes, after heating, its waveguiding properties, loosing the capability of coupling light between the part and the other waveguide core. For example, the refractive index difference of the part can be reduced to about half the refractive index difference of the other waveguide core, the refractive index difference being taken as the difference as described above between the refractive index of the considered part or core and that of the bulk.